OF THE FIXED STARS. 121 



ill the hravrns, and shine with its present brilliancy for twelve years to 

 come. 



While Struve was engaged with these observations on Vega, Profes- 

 sor Bessel, of Konigsberg, was observing 61 Cygni for the same pur- 

 pose. This is a star of the fifth magnitude, about seventy-eight de- 

 grees east from Vega. It was chosen, because, being a double star, it 

 could be observed with the greater accuracy ; and because of its grc^at 

 proper motion, which amounts to about live seconds a year. Its two 

 component stars, moreovei*, have the great angular distance of sixteen 

 seconds ; both which circumstances indicate its comparative nearness 

 to our system. The Professor continued his observations with un- 

 wearied industry from 1837 to 1840. Near 61 Cygni he selected two 

 small stars which do not partake of its proper motion, one situated in 

 the line of direction of its two component stars, the other perpendicu- 

 lar to this line. With his great Frauenhofer heliometer he measured 

 the varying distances of these stars from the point which bisects the 

 distance between the components of the double star. From a series of 

 the most accurate and laborious observations, this celebrated astronomer 

 determined the annual parallax of 61 Cygni, to be 0."348, or a little 

 more than one-third of a second. Consequently its distance is •5&2,000 

 times the earth's mean distance from the sun, or 56,240,000,000 miles; 

 a distance which would not be traveled by light in less than nine years 

 and four months. 



Tiie distance of some of the fixed stars being now ascertained, we 

 may confidently anticipate the determination of the distances of others, 

 and perhaps eventually their magnitudes and densities. The only diffi- 

 culty which presents itself in estimating the diameter of a star, the dis- 

 tance of which is known, arises from the fact that, generally, their 

 discs, even when viewed by telescopes of the greatest magnifying pow- 

 er, do not subtend any appreciable angle, but appear merely as luminous 

 points. Sir William Herschel states, however, that he found tlie appa- 

 rent diameter of Vega to be 0."3'5-53 If this be correct, its real diame- 

 ter is about 136 times greater than that of the sun, and, consequently, 

 its magnitude more than 2,500,000 times greater. 



The distance of any two stars from the earth being known, their 

 distance from each other may also be readily determined. The two 

 components of 61 Cygni, for example, having an angular distance of 

 sixteen seconds, their real distance from each other is found to be about 

 4,750,000,000 miles. If, therefore, their masses arc C(|ual, the circum- 

 ference of the orbit which each describes, is 14,922,600,000 miles. 

 Now the period of revolution of this star has been estimated by Her- 

 16 



